Solvent-based coatings with a thermoset resin as a main component have been conventionally applied to the inner face and the outer face of metal cans for food for the purpose of preventing the corrosion of metal can materials while maintaining the taste of contents or for the purpose of improving the appearance of the outer face of cans, protecting printed faces, and the like. However, the solvent-based coatings are required to be heated at high temperatures in order to form coated films, which vaporizes a large amount of solvents during the process, leading to problems in terms of operational safety and environment. Given these circumstances, the coating of metal with a thermoplastic resin has been recently developed as a solvent-free method of corrosion prevention. In this technique, polyester in particular is excellent in formability, heat resistance, and the like among thermoplastic resins, and polyester-based films to be laminated on metal are being developed.
However, metal containers laminated with polyester films have a problem in that their appearance and the taste of contents are impaired during high-temperature sterilization treatment such as retort sterilization treatment. There is another problem in that whitening (retort whitening) occurs, in which the resin layer itself discolors to be turbid in white during the retort sterilization treatment.
To address the above circumstances, many techniques are disclosed as a method for maintaining the taste of contents. Patent Literature 1 to Patent Literature 6 disclose a resin coating technique with a butylene terephthalate unit as a main body, for example.
In particular, a decorative film for a steel sheet disclosed in Patent Literature 4 includes an adhesive layer with a noncrystalline copolymerized polyethylene terephthalate-based resin as a main component on its face to be laminated on a steel sheet. In this adhesive layer, part of ethylene glycol as a diol moiety of polyethylene terephthalate is replaced with 1,4-cyclohexane dimethanol. A layer with a pigment-added polybutylene terephthalate as main component is provided on the adhesive layer. This adhesive layer has a thickness of 2 to 50 μm that is less than 50% of the thickness of all the layers.
Patent Literature 5 discloses a laminated, polyester film including at least two layers, the film mainly for a wallpaper surface layer. One layer of polyester out of the layers is a film excellent in gas barrier properties containing 90 mol % or more of an ethylene glycol component and an 1,4-butanediol component as glycol components.
A resin-coated steel sheet for a container disclosed in Patent Literature 6, which is a steel sheet for a 18-liter can, is coated with a resin layer obtained by blending polybutylene terephthalate and polyethylene terephthalate together so that the resin layer is in contact with the steel sheet. A layer formed of polybutylene terephthalate is laminated thereon to form two layers, and a layer formed of a copolymerized polyester resin containing polyethylene terephthalate and isophthalate is laminated further thereon. Out of the former two layers, the layer applied so as to be in contact with the steel sheet is an unoriented layer in which the blending ratio of polybutylene terephthalate is 1/5 or higher relative to polyethylene terephthalate, and the layer formed of polybutylene terephthalate laminated thereon is an unoriented layer.
Patent Literature 7 and Patent Literature 8 disclose films formed of polybutylene terephthalate and polyethylene terephthalate.